feat: add title_font_size query param

[ToledoVitor]

Closes #139

Summary

• Adds a title_font_size query param (5-40 inclusive, default 20 — matches the previous hardcoded value, so existing embeds render unchanged).
• The title's foreignObject box scales with the font size so larger titles don't get clipped.

All Submissions

• Have you followed the guidelines in our Contributing document?
• Have you checked to ensure there aren't other open Pull Requests for the same update/change?

New Feature Submissions

1. Does your submission pass tests?
2. Have you lint your code locally prior to submission?

Test plan

• npm test passes locally (existing suite + 2 new assertions covering clamping and a custom font size)
• tsc --noEmit and prettier --check clean
• Verified visually at the real production render width (1200px) across the full 5-40 range with default and custom titles — no clipping, default output byte-identical to before

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